1. Field of the Invention
This invention relates to novel foamed rubber compositions for pneumatic tires and a method of producing the same, and more particularly to a foamed rubber composition suitable for use in a tread of a pneumatic tire, in which each of the resulting closed cells in a matrix rubber is covered with a coat layer, and a method of producing the same.
2. Description of Related Art
Recently, the demand for so-called all season tires capable of using without a tire exchange even in winter season likewise summer season is increasing. This type of the all season tire develops dry gripping property, wet gripping property, steering stability, durability and low fuel consumption even in winter season likewise summer season, and further has sufficient traction and braking performances on snow and/or ice roads.
As a tread rubber used in the all season tire, there have hitherto been known a method of decreasing a hardness at a low temperature in a tread rubber for summer season, a method of using a polymer having a low glass transition temperature or using a softening agent capable of adequately holding a modulus of elasticity of the tread rubber at a low temperature, and the like.
In the former method, however, the performances at snow and ice temperature range are almost developed owing to the hysteresis property of the polymer, but there are problems that the braking performance and steering stability on wet and dry roads are insufficient. On the other hand, the latter method is disclosed, for example, in JP-A-55-135149, JP-A-58-199203, JP-A-60-137945 and the like, but has a serious problem that it badly affects the wear resistance and durability in the running on general-purpose road though the performances on snow and/or ice roads are improved.
In any case, the tread rubbers produced by the above methods certainly exhibit good performances at a relatively low temperature range of not higher than -5.degree. C. or at so-called dry-on-ice state, but do not provide sufficient friction coefficient at a wet state of about 0.degree. C. or at so-called wet-on-ice state, so that it can not be said that the traction performance, braking performance and steering stability are sufficiently improved.
Lately, there are adopted some processes of applying means for the improvement of friction force to the tread rubber composition itself. As a first process, there is a process wherein the tread rubber is foamed by a proper means to form closed cells in the tread rubber composition (JP-A-63-89547). In general, a layer called as a false liquid layer is existent on a surface of an ice. When an object is forcedly moved on the ice surface, a part of the false liquid layer changes into a water film, which acts as a lubricant to develop a low friction constant. In the first process, the surface of the thus obtained tread rubber are covered with a great number of cells, so that the removal of water film created on a zone contacting with ice surface and the edge action of scraping the false liquid layer accompanied with a microscopic movement of the cell are developed to provide high friction properties on ice. The tires manufactured by using this process are commercially available as a studless tire. However, they have the effect of removing the false liquid layer from the ice surface, but can not directly scrape or break the ice itself different from the conventionally used spike tire, so that the improvement of the performances on ice is not yet satisfied.
As a second process, there is a process for attaining the high friction properties on ice by incorporating various high-hardness materials into the tread rubber to utilize the scratching effect of these materials on ice surface (JP-B-46-31732, JP-A-51-147803, JP-B-56-52057 and JP-B-6-102737). This process is apparent to be a process for providing the high friction properties of the tread rubber on ice by a mechanism different from the above first process. In fact, as the amount of the high-hardness material incorporated becomes larger, the tread rubber tends to provide the high friction properties on ice.
In the first process, the surface of the tread rubber is rendered into a rugged surface by foaming, so that ice surface is scratched by convex portions of the rugged surface and water existing on ice surface is absorbed and discharged by concave portions of the rugged surface. Moreover, the water absorbing and discharging effect is not required at a low temperature range hardly melting the ice surface (not higher than -3.degree. C. in usual case), but the scratching effect can not be expected too much at such a low temperature range.
In the second process of incorporating the high-hardness material developing a high scratching effect into the matrix rubber of the tread, the effect of improving the performances on ice at about 0.degree. C. having a large water content is small at this temperature. Further, the high-hardness material is existent as a foreign matter having no affinity with rubber, so that the wear resistance and fatigue properties are considerably degraded.
Under the above circumstances, the inventor has examined the incorporation of composite particle consisting of syn-1,2-polybutadiene resin, sulfur, vulcanization accelerator, carbon black and scorch retarder into a foamed tread rubber in order to simultaneously establish the improvement of performances on snow and/or ice road surfaces at wet state and the wear resistance and confirmed that the use of such a composite particle is not yet sufficient in a level required for commercial markets. That is, it is demanded to further improve the composite particle.
Moreover, the actual ice surface temperature variously changes over a time of from day to night, so that it is strongly desired to develop a tire tread indicating more stable performances on ice at a wider temperature range and causing no degradation of wear resistance and fatigue properties.
On the other hand, the conventional foamed rubber has a function of improving the friction coefficient on ice by the dewatering and water discharging effects of flowing out water film generated between a ground contact region of the tire and an ice road surface. However, such effects are critical in order to remove a greater amount of water film at the wet-on-ice state, so that it is difficult to improve the friction coefficient on ice at the wet-on-ice state.
Alternatively, the performances on ice are improved to a certain extent by incorporating particles having particular particle size and hardness as microspike for developing the scratching effect into the tread rubber, but the synergistic action between the scratching effect and the water discharging effect is less, so that the level of the performances on ice is still insufficient. That is, the feature that the synergistic action is small is due to the fact that the hardness of the particle is in inverse proportion to the adhesion property of the particle to rubber. In the conventional technique, as the particle becomes harder, the adhesion property of the particle to rubber lowers though the scratching effect is improved. As a result, the particles are apt to be fallen out from the ground contact region of the tread at a friction face between the tire and the road surface and hence the number of effective microspikes undesirably reduces and the scratching effect is not held long and also there are problems that the wear resistance and crack resistance are degraded. When the hard particles are added to a rubber composition containing a foaming agent, the foaming is caused at boundary surfaces of the particles to lower the adhesion force to matrix rubber.